A Look at SÉCAM III

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Open Source (CC BY 4.0 ), Proprietary (CC BY-ND 4.0 ) Scrolling past this page is The Point of No Return and Acknowledges that You have Agreed to the Terms above. If you are Unable or Unwilling to Agree to these Terms then Close this Document. Copyright ©2018 – J. S. Gilstrap – All Rights Reserved. A Look At SÉCAM III SÉCAM (Séquentiel Couleur Avec Mémoire) is a color TV system developed in France. This was a joint project with the USSR as Soviet technicians worked alongside French engineers to produce what was to become SÉCAM III along with some additional improvements later to be known as the 'A' and 'B' suffixes. The Soviets later came up with another version called SÉCAM IV that was incompatible with SÉCAM III but more on that later. Instead of using QAM to transmit the two channel color information for every scan line as it is done in NTSC and PAL the two color signals are transmitted on alternate lines. To recover the non-current line of color information a delay line of 1H is used to recover the other color from the previous line, hence the part 'Avec Mémoire' or 'With Memory'. The delay line is used after the detected signal so critical delay timing is not an issue as it is for NTSC or PAL delay lines. The R G B weighting for the Luminance is the same as Y I Q in NTSC and Y U V in PAL but the color matrix uses different scaling factors called Y' DB DR. Since each color channel is transmitted separately the B─Y and R─Y signals are scaled so they both peak at ±1⅓ to maximize modulation for both channels. If displayed on a vector scope the hexagon is more uniform in shape compared to NTSC's or PAL's tall and narrow pattern. It is rumored but unconfirmed that this scaling might be used for PAL-N which could provide better automatic hue correction on marginal signals although increased U levels could cause over modulation issues if not properly addressed. The color signal is also modulated on an FM sub-carrier instead of AM and offers distinct advantages but some disadvantages too. FM by its very nature has better noise immunity than AM and the limiting process eliminates any AM noise. If the FM detector operates above the limiting knee then full quieting is achieved as the signal has captured the detector. Good design must go into the detector in order to produce a good capture ratio. Below the limiting knee when the detector is not fully captured a not-beautiful 'SÉCAM Fire' will flair up and spread across the whole scene. In NTSC or PAL the BFO signal is provided by a PLL crystal oscillator so the reference signal is always there unlike an FM detector seeing a signal below the limiting knee threshold level. Under these conditions the detector will act erratic and produce a false signal. Both NTSC and PAL have color killer circuits when the PLL is not locked so an improperly detected color signal is not available. Higher end SÉCAM sets probably have something similar but momentary loss of a captured detector from noise in the middle of a picture will produce saturated color streaks on the screen as will text inserted from VCR/DVD menus as this squashes the FM sub-carrier. If improper detection of line switching order and/or phase occurs then the whole color palate can flip and colors can become completely different. The FM sub-carriers are always present unlike QAM-SC used in NTSC and PAL. In B & W areas the two sub-carriers are reduced to 23% of their levels during full color saturation. This is done through the use of an inverted Bell Curve notch filter that has maximum attenuation when the sub-carriers have no color modulation. As the frequency of the sub-carriers shift they move out from under the center of the notch filter introducing amplitude modulation. While the unmodulated FM sub-carriers are a multiple of the line frequency maintaining them in exact sync with the rest of the signal is nearly impossible. This and along with the character of FM prevents any composite mixing of two or more signals or fading from one to the next. SÉCAM IV developed by the Soviets at NIR had some different characteristics. It was non- linear, assuming in its deviation be it frequency or phase. It was incompatible with SÉCAM III. There were two versions, one with and one without gamma correction. Prior to their venture into SÉCAM they experimented with what would seem to be a version of NTSC Chroma using the European 625 line B & W D/K system called 'Simultaneous System with Quadrature Modulation'. Supposedly there were even TV sets manufactured using this system listed in the goods catalog. From working with this system and being familiar with QAM it wouldn't be surprising if one of these sets became a SÉCAM IV conversion. The envelope of both the sub-carriers is √Db²+Dr² +10%pep, carriers on even[odd] lines within a field are phase modulated with aTan[DB/DR] while the odd[even] lines are unmodulated. A very accurate 1H delay line is used to supply the two carriers to a product detector. Recovering both signals might depend on the phase offset of the BFO, 0° or 90°, applied to the detectors. This is overly complex, inherently flawed and probably why it was rejected. Using plain Armstrong PM would be a much simpler approach and an easier conversion. Use only one of the QAM detectors output, add a delay line, a line switching detector with switch, and that should do it. For transmission replacing one of the QAM color signals with a DC level for the carrier which would be in quadrature to the DSB-SC AM modulated signal would produce non-linear Armstrong Phase Modulation. This single modulated signal would switch between DB & DR on alternate lines. If the modulation index peaked at ±1⅓, which are the DB DR peak values, then this would produce a non-linear phase deviation and apply a gamma. It would also introduce inverse Bell Curve amplitude modulation to the signal that would be 23½% at zero crossing in relation to its peak ±1⅓ modulation index. What a coincidence, SÉCAM III is spect. at 23%. If proper equalization and gamma correction were applied then this would produce an FM compatible signal with a linear frequency deviation that FM discriminators could properly detect. If the carrier was an odd multiple of ½ the horizontal line frequency then the clusters and fine mesh harmonics would interleave just like NTSC and the dot crawl pattern would be identical when no color modulation was present. The dot pattern could also have a two frame repeat rate depending on the line switching scheme. Since the color sub-carriers would stay synchronized with the horizontal frequency two or more signals could be mixed together or faded from one to another, although adding gamma correction for a linear deviation would negate this feature for Chroma signal modulations above 40% of peak producing inaccurate color mixing, hue and saturation changes.

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